1. Field of the Invention
In general, the present invention relates to measurement devices used to properly fit eyeglasses. More particularly, the present invention relates to instruments used to determine the center of an eye's pupil for use in fabricating multi-focal lenses.
2. Prior Art Description
Eyeglasses can be either framed or frameless. Framed eyeglasses have lenses that are mounted on a frame made, for example, from an aluminum or titanium alloy or some other material. Frameless glasses have lenses that are only secured by a nosepiece and the ear handles of the eyeglasses.
Eyeglasses can also have either single-focal lenses or multi-focal lenses, so-called progressive lenses or multi-focal lenses. When the wearer is at the opticians buying a new pair of such eyeglasses, certain centering data has to be recorded in order to fit the new lenses. This data is necessary for the grinding of the lenses and for when they are fitted onto their frames, even in the case of frameless glasses. In order to ensure that the multi-focal lenses correspond to the wearer's individual requirements, the manufacturers of such multi-focal lenses need to know a point on the lens directly in front of the pupil when the wearer is looking straight ahead and is said to have a natural posture and head position. When it comes to measuring the height of the middle of the pupil in relation to the lowest part of the spectacle frame, or in the case of frameless glasses in relation to the lowest part of the lens, the normal approach is that the optician will stand in front of the wearer and ask him or her to look straight ahead. At the same time, the wearer will be requested to take on a natural posture and head position. The optician will then try to sight the middle of the pupil and mark it on the lens using a felt marker. In the case of a new pair of spectacles, an optician will mark the demo glass.
There are a number of disadvantages of this conventional approach. If the wearer were to take on a posture and head position that is different to his natural one, a wrong measurement can be recorded. For example, if the wearer tilts his head further back than normal, this causes him to look through a lower part of the lens in relation to the lowest part of the eyeglasses. The optician would then incorrectly sight the position of the middle of the pupil because of the wearer's unnatural posture and head position. If the lenses of the eyeglasses are made using this incorrect measurement, the eyeglasses would be incorrect.
It has been observed that a person often holds his/her head in an unnatural position when being fitted for eyeglasses. When the optician stands in front of a person to take a pupil measurement, that person often assumes a posture and head position that is different to his natural one. For example, a person may stand far more erect that he normally would. Alternatively, a person may tilt his head further back than normal. The incorrect measurements may render the manufactured eyeglasses unusable, with the result that the optician has to redo the measurements and replace the lenses.
Another problem resulting from this conventional approach is that in order to mark the spectacle lens, or the demo glass, the optician has to have the same eye height as the customer in order to avoid the problem of vertical parallax. This, in turn, is only possible if the optician himself, or a member of his staff, is of the same height as his customer. Since this is not usually the case, the customer is asked to sit down so that the person taking the measurements can sit opposite the customer and simulates the same height. Even if, when seated as described, the person taking the measurements is of the opinion that they are at the same eye height as the wearer, this is still subjective, because there is no proof whatsoever that the eye height of the wearer is the same as the eye height of the person taking the measurement. Relevant observations have shown that a difference in eye height, i.e. in the height of the middle of the pupil, of some 5 cm is typical, thus leading to the above mentioned problem of vertical parallax. This means that the inaccuracy with which the distance between the middle of the pupil and the lowest part of the eyeglasses is measured is some 1.1 mm while an accuracy of +/−0.2 mm is required.
In the prior art, measurement devices have been developed that are used in the proper fitting of eyeglasses with multi-focal lenses. For instance U.S. Pat. No. 4,505,043 to Sztuka, entitled Height Measurement Gage For Multifocal Lenses, a measurement device is shown that attaches to the frame of a pair of eyeglasses. The device records the position of the pupil relative the eyeglass frame. This eliminates the need to mark the lens with a felt marker. However, the device is only suitable with eyeglasses having specific angles of inclination. Furthermore, the device does not eliminate errors caused by taking measurements while a person's head is in an unnatural position.
U.S. Pat. No. 5,640,775 to Marshall, entitled Determining And Marking Apparatus And Method For Use In Optometry And Ophthalmology, discloses a measuring device where a person positions a light between his/her eye and a distant target. The light theoretically shows the person's line of sight. The point where the light passes through the eyeglass lens is marked and used in the making of the multi-focal lenses.
The holding of a light source in front of the eye is an odd act that causes people to cock, tilt, and otherwise hold their head in an unnatural position as they attempt to align the light and a distant target. Consequently, the device of the Marshall patent does not provide a system that creates a measurement while a person is holding his/her head in a natural position and posture.
A need therefore exists for a device that can be used to accurately measure the position of a person's pupil in relation to a pair of eyeglasses, without having a person move his/her head from a natural position and posture. This need is met by the present invention as described and claimed below.